Mechanical removal of pins in a confined space for access is typically performed by means of hand drills or other cutting machinery such as mills. Such removal is in many circumstances difficult, time consuming, and costly.
For the maintenance of machines, such as motors and turbines, the space for access to particular machine parts such as pins with such tools is confined. The maintenance frequently requires difficult manual labor and individual handling of maintenance operations within the given confined space. Alternatively, in order to provide larger space for access, the machine is transported to another facility and disassembled. Either method of maintenance is costly not only due to its labor intensity but also due to the long downtime of the machine being out of operation.
Steam turbine blades are attached to the rotor, among others, by means of a straddle-type blade root and press-fit pins placed in boreholes extending through the blade root and rotor. Prior to their placement in the boreholes, the pins are for example cooled to low temperatures, e.g. by means of liquid nitrogen. Thus slightly reduced in size, they are then pressed into the borehole with heavy-duty tools, which results in a tight, high-tension press-fit between the pin and the turbine rotor and blade root.
During turbine maintenance, the turbine blading must be removed and replaced requiring the removal of the press-fit pins from their boreholes. However, this is a difficult procedure because the space between the blade rows is confined, in some cases as narrow as ten centimeters.
In a first known turbine maintenance method, the space for access is increased by removing and transporting the entire rotor to a manufacturing or maintenance facility and segmenting it into several parts. The pins are then removed using heavy-duty drills or electro discharge machining (EDM) drills. The latter must be used with great care as the heat developed in the EDM drilling can cause a soldering of the pin to the rotor.
In a second known method, the pins are removed “in situ” thus avoiding transporting and segmenting the turbine rotor. Each pin is removed individually using standard, handheld drills with hard metal or diamond drill bits. Particular care must be taken in order to prevent any damage to the material surrounding the borehole so that a high quality press-fit of the pin fastening the new blade to be mounted can be assured.
Both methods, rotor segmentation and “in situ” removal, are time-consuming and costly procedures.
GB 2 349 111 discloses a general use of water jets (as opposed to EDM), for drilling of cooling air holes in turbine airfoils. The cooling air holes are drilled prior to mounting the airfoil on the rotor. The application thus has no particular access space requirement.
US 2005/0050706 discloses the use of a water jet for the cutting of a rivet in steelwork. The jet is used to cut a circular ring in the rivet in order to extricate the rivet. The water jet apparatus comprises an extensive assembly of large size and thus unsuitable for drilling in a confined space.